Many machines and mechanisms operate in a cyclical manner. For example, rotating machinery such as turbines rotors, and reciprocating mechanisms such as paint shakers, exhibit cyclical motion. In use, these machines and mechanisms may be exposed to varying load conditions. However, many cyclically-operated machines and mechanisms are not able to accommodate varying loads while maintaining desired performance without substantial increases in power consumed. A need therefore exists for a simple, efficient system for driving cyclical machines and mechanisms, and for accommodating varying load conditions.